The goal of this proposal is to analyze the developmentally regulated expression and function of two Dictyostelium cell surface cAMP receptors, CAR1 and CAR2. Particular emphasis is placed on defining the roles of these G-protein coupled receptors late in development, a time when they are both expressed. Based on apparent structural differences, it is hypothesized that the two receptor types mediate different functions, possibly resulting from differences in G-protein coupling and/or spatial differences in expression. Classical and molecular genetic approaches are combined to test this hypothesis. The genes for these receptors will be manipulated in vitro and returned to their normal developmental context. That means that answers derived about their signalling mechanisms and specific developmental functions can be interpreted directly without the necessity of heterologous expression studies. More than 80 different G- protein coupled receptors have been identified and defects in several G- protein linked systems have now been associated with disease states. G- protein coupled signaling is also being found to be important in vertebrate development suggesting that this mechanism of signal transduction is used in a wide variety of contexts. Dictyostelium provides an excellent system in which to ask questions about the importance of G-protein coupled signalling in development. The fact that the developmental system is relatively simple, that genes for available makes sorting out the roles of the individual players an approachable task. Because of the remarkable structural conservation seen among G-protein coupled receptor systems, it is reasonably hoped that information acquired here will have broader applicability. The experiments described here are founded in promising preliminary results and the questions made feasible by the combined developmental, molecular, and genetic advantages of Dictyostelium as an experimental system.